In the prior art, optical systems have been disclosed, such as for example in U.S. Pat. No. 4,065,786, issued on Dec. 27, 1977 to W. C. Stewart, for directing light from a coherent light source upon an information track, formed as a succession of depressed areas of a given width alternating with non-depressed areas along the length of the track, with the light focused on the track as a spot having a half-intensity width (i.e., the dimension of the spot in a direction transverse to the length of the track, measured between half-intensity points) which is less than the track width (i.e., less than the given width of the depressed areas of the track).
The spot width choice of the aforementioned prior art systems is in contrast with the spot dimension employed in other prior art systems of the type exemplified by the arrangement described in the article entitled, "The Optical Scanning System of the Philips `VLP` Record Player", by G. Boushuis and P. Burgstede, appearing at pages 186-189 of volume 33 of the Philips Technical Review (1973, No. 7). In the Philips arrangement, a circular light spot focused on the information track of a reflective video disc has a half intensity width which is greater than the width of the depressed areas of the information track by an amount sufficient to ensure that a significant portion of the incident light falls upon the adjoining "lands" (flat disc surface areas between adjacent track convolutions) when the spot is optimally centered upon a desired track convolution.
In either of the above-mentioned optical record playback systems the focused light spot provided at the record surface (i.e., the narrowest waist of a light beam) is commonly obtained at the focal plane of a focusing lens which has been illuminated by a monochromatic light beam (hereinafter referred to as full area illumination). This narrowest beam waist can be generally characterized as having a diameter at its half-intensity width of the size .lambda./2a, where .lambda. is the wavelength of the monochromatic beam and a is the numerical aperture of the lens. The maximum axial deviation distance from the narrowest beam waist at which the beam waist still maintains its general dimensional characteristic is referred to as the depth-of-field and is equal to .lambda./a.sup.2. It will be appreciated that during playback of an optical record it is desirable for satisfactory playback of recorded information to maintain deviations in the distance between the focusing lens and the record surface within a range of values determined by the aforesaid depth-of-field distance. It will be further appreciated that since the depth-of-field distance is directly related to the half-intensity width of the beam waist at the focal distance, difficulties in maintaining an appropriately focused spot on the record surface are increased with the decrease in spot size.